Solenoid actuators are well known in fluid flow control, where an orifice or aperture in a fluid flow path is to be opened or closed by means of a closure member such as a plunger, rod, spool or the like. Such actuators commonly comprise a magnetic circuit including a flux-generating coil, and a plunger formed of magnetic material moving under the influence of a magnetic field that changes in response to varying current flow through the coil. The plunger may be mechanically coupled to the closure member, which opens or closes the aperture or orifice in the fluid flow path as the plunger moves in accordance with changes in the magnetic field.
In a particular arrangement, it is known to use a solenoid actuator as part of a camshaft control system (i.e., cam phaser position control). As known, the camshaft of an internal combustion engine may be employed to control the opening/closing of engine valves (e.g., intake, exhaust). As further background, then, cam phasing may be understood as the shifting of valve events in a crank angle (or cam angle) domain. Typically, a mechanical device is attached to the end of the camshaft for such purpose (“cam phaser”). The cam phaser may include an oil-actuated piston coupled to a gear train, a spool control valve for controlling the flow of oil to the piston, and an actuator for controlling the spool control valve. The actuator is driven by a pulse width modulated (PWM) signal from an engine control unit. The actuator includes a forward rod that extends into the spool valve and acts as a closure member, opening/closing various ports. As the duty cycle of the PWM signal is varied, the rod is caused to move to a controlled depth in the spool valve, controlling the flow of oil, for example, to one side or the other of the above-mentioned piston, thereby in-effect actuating the gear train in a controlled fashion. The gear train moves the camshaft. FIG. 1 is a cross-sectional, side view showing a conventional solenoid actuator 10 used in connection with the above-described cam phaser. Actuator 10 includes a solid plunger 12 having a rod portion 13, a cup 14, a magnetic coil 16, a secondary plate 18, a washer 20 and a bobbin 22. Plunger 12 is a solid piece of ferromagnetic material, conical in shape at the front or at the tip, and is typically machined from a steel bar. Plunger 12 is also shown to include a plurality of axially extending flutes 24 formed in the outer surface of plunger 12. Flutes 24 interact with the oil for damping and hydraulic force compensation. FIG. 1 is taken in section through a pair of such flutes; thus, flutes 24 shown in FIG. 1 are shown without cross-hatching. Cup 14 acts as a guide for plunger 12 and additionally isolates plunger 12 from secondary plate 18. Cup 14 is a deep drawn component that is supported inside bobbin 22 to avoid fractures of the cup itself. Washer 20 is used as a magnetic brake between plunger 12 and primary plate 18. In operation, plunger 12 moves within cup 14 in accordance with the magnetic force induced by the magnetic flux produced by coil 16. Rod portion 13 is configured to extend into a spool control valve (not shown) for controlling oil flow, as described above. However, there are several shortcomings.
First, the rod portion 13 of the integral plunger/rod 12 may become bent during the manufacturing operation (e.g., machining) or shipping, which may result in an inoperable actuator when assembled and tested. Second, the flutes, among other features, are relatively complex to manufacture. Third, the inside diameter surface of cup 14 (i.e., the guide) and the outside diameter surface of plunger 12 (i.e., the guided part) are coextensive over a relatively large area, thus increasing drag or friction therebetween. Fourth, a hydraulic lock (i.e., sticking) condition may occur when the rear of plunger 12 contacts the closed end of cup 14.
There is therefore a need for an improved actuator that minimizes or eliminates one or more of the shortcomings set forth above.